1. Field of the Invention
The present invention relates to a stage system and a lithographic apparatus comprising a stage system.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The known lithographic apparatus comprises a movable stage system configured to support a substrate. The stage system is movable to position the substrate supported thereon in six degrees of freedom with high accuracy. The main plane of movement of the stage system is parallel to the main plane of the substrate, for example the horizontal plane, although the stage system is usually also capable of moving the substrate over a small range in the direction perpendicular to the main plane of the substrate, e.g., the vertical direction.
The stage system may comprise a substrate table configured to support the substrate and an encoder block. At its bottom side the substrate table comprises a large number of burls to be arranged on a substrate table support surface of the encoder block. The substrate table is clamped by means of a vacuum clamp and/or electrostatic clamp on the substrate table support surface. The clamping force should be sufficient to maintain the substrate table in the same position with respect to the encoder block. The normal force, i.e., the force with which the substrate table is pressed against the encoder block multiplied by the friction coefficient should be larger than the shear force in the burls introduced by acceleration of the substrate table and encoder block and/or by deformation of the encoder block to avoid (local) slip between the substrate table and the substrate table support.
The encoder block comprises actuators to actuate the encoder block in two or more directions and an encoder system to measure the position of the encoder block. Since the position of the substrate table and the substrate supported thereon is measured based on the position of the encoder block, it is undesirable that the substrate table moves with respect to the encoder block, as movement of the substrate table with respect to the encoder block may result in misalignment of the substrate with respect to the patterned beam. Such misalignment may cause overlay errors.
In the prior art stage system Lorentz actuators are used to drive the encoder block in a driving direction. The Lorentz actuators are arranged in the driving direction serially with respect to the substrate table, i.e., see in the driving direction before and after the substrate table. As a result, the load in the driving direction of the Lorentz actuators on the support table support surface will result in hot spot peaks of material stress in some burls of the substrate table.
These hot spot peaks of material stress may locally have a negative influence by having increased local shear forces and reduced normal forces on the substrate table which may result in local slip between the substrate table and the substrate table support when acceleration levels become too high.
To increase throughput of a lithographic apparatus, it is desirable to increase the acceleration with which the substrate stage is accelerated to the scanning speed. However, the local slip effect between the substrate table and the substrate table support limits the possibilities of increasing the acceleration levels used in the stage system.